Excellent short list! They are all lovely. From your list, I would (with difficulty) select the following five:

Ceridwen: the Welsh goddess of poetry is a rich, evocative namesake, with a beautiful, lilting, and still-used name. It's crisp and lovely to say, and I think of your choices it's by far the most commonly given. Its phonetics are different from both your other girl choices as well as your boy options.

Hespera: the mythological and poetic history of this name is simply too wonderful. I love the 'nymph of the evening,' and the astronomical associations. It's also easy to say, spell and pronounce. Again, it sounds a bit different from the other names you're considering.

Illyria: I admit to this being my favorite. Half-real, half-shrouded in myth and legend; Shakespearean, Byzantine, and Utopian. It absolutely flows off the tongue.

Niniane: The Lady of the Lake, with a cool, flowing name reminiscent of the waters.

Tinuviel: a Tolkien name that only fanatics will really recognize; others will just see its gorgeous aesthetics and phonetics. Again, the Elvish meaning "daughter of the starry twilight" is so beautiful. Again, the sound is distinctive (esp if she gets brothers ending with -on).

The two I didn't select:

Belphoebe: I know this is a top contender, and I like its Spenserian origins (I think she was a great namesake). But I think the modern world won't recognize it for the beauty it is, and will insist on seeing it as a smash between Bella and Phoebe. Pearls before swine, if you want to be snooty.

Evelake: It's beautiful, but the namesake was a half-mythological Welsh king who didn't do much. I would perhaps move it to middle name territory?

I have to say that you are brave! I have been following your posts to see what names have made the cut! Your names all make me want to run through a meadow chasing fairies, and I mean that as the biggest compliment! My favorite is Niniane. I think it's beautiful and fun and perfect. If I met a child named Niniane, I would want to know more about her and I would definitely want to meet her parents.

Belphoebe: I know this is a top contender, and I like its Spenserian origins (I think she was a great namesake). But I think the modern world won't recognize it for the beauty it is, and will insist on seeing it as a smash between Bella and Phoebe. Pearls before swine, if you want to be snooty.

Oh I hate the rolling pearls before swine argument against Belphoebe! Belphoebe is so much more fairylike and beautiful than just Belle or Phoebe smushed. I say it and I can hear Purcell playing in my ears (I know, different fairy queen.)

My top 4, in order of preference:
Hespera - Love this with Endymion. Both names have such heady, nocturnal imagery connected to them.
Belphoebe
Illyria
Niniane